How Machines Work

What Is a Machine?

A machine is a tool that helps us do work. In science, "work" means using a push or a pull, called a force, to move something.

Machines do not give us magic power. Instead, they are clever helpers. They let us lift heavy things, cut hard things, or move objects far away using less effort than we could with our hands alone.

Some machines have hundreds of parts, like a car. But every big machine is built from a few simple ideas called the six simple machines. Let's meet them all!

The Lever

A lever is a stiff bar that rests on a turning point called a fulcrum. When you push down on one end, the other end lifts up.

You use levers all the time:

• A seesaw in the park is a lever.
• A crowbar that pops open a lid is a lever.
• Even your arm works like a lever when you lift a cup!

A lever lets you lift something heavy with a small push, as long as you push from far enough away from the fulcrum.

The Wheel and Axle

A wheel and axle is a round wheel joined to a rod in the middle, called the axle. When the wheel turns, the axle turns too.

Wheels make moving things much easier. Instead of dragging a heavy box across the floor, you can roll it on wheels with hardly any effort.

You can find wheels and axles in:

• Cars, bikes and skateboards
• A doorknob you twist to open a door
• The wheels on a shopping cart

The Pulley

A pulley is a wheel with a groove in it, with a rope running over it. When you pull the rope down, the load goes up.

A pulley changes the direction of your pull. Pulling down is easier than lifting up, because you can use your own weight to help.

Pulleys are used to:

• Raise a flag to the top of a flagpole
• Lift heavy loads on a crane at a building site
• Open and close window blinds

If you use several pulleys together, you can lift very heavy things with a gentle pull.

The Inclined Plane

An inclined plane is just a fancy name for a ramp — a flat surface that is slanted, with one end higher than the other.

Pushing a heavy box straight up is very hard. But pushing it up a ramp is much easier, because the ramp spreads the work over a longer, gentler slope.

You see inclined planes in:

• A wheelchair ramp
• A slide at the playground
• A road that climbs slowly up a hill

The Wedge

A wedge is like two inclined planes joined back to back to make a sharp edge. A wedge is used to push things apart or cut things.

When you push a wedge into something, its slanted sides force the object to split open.

Wedges include:

• The blade of a knife or an axe
• Your front teeth, which cut into food
• A doorstop that wedges under a door to hold it still

The Screw

A screw is an inclined plane wrapped around a pole, making a spiral ridge called a thread.

When you turn a screw, the spiral pulls it into the wood and holds it tightly. A screw turns a turning motion into a strong forward push.

Screws are used in:

• The screws that hold furniture together
• The lid of a jar that twists on and off
• A spiral staircase, which is a giant screw you can walk up!

Putting Machines Together

Most machines you see every day are compound machines — they combine two or more simple machines.

Think about a bicycle. It has wheels and axles to roll, levers in the brakes and pedals, and screws holding the frame together. A pair of scissors uses two levers and two wedges (the blades) at the same time!

When you understand the simple machines, you can spot them hiding inside almost any tool or vehicle.

What We Learned

Machines make hard work easier. There are six simple machines: the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw. Big, complicated machines are just lots of these simple ideas working together.

Next time you open a door, ride a bike, or cut your food, look closely — you'll find a simple machine doing the work!

Curious about how clever inventions changed the world? Read A Short History of Flight, or explore the wonders of space in Understanding Our Universe.